1.5 Types of Database Systems

Database Systems can be catagorised according to the data structures and operators they present to the user. The oldest systems fall into inverted list, hierarchic and network systems. These are the pre-relational models.

  • In the Hierarchical Model, different records are inter-related through hierarchical or tree-like structures. A parent record can have several children, but a child can have only one parent. In the figure, there are two hierarchies shown - the first storing the relations between CUSTOMER, ORDERS, CONTACTS and ORDER_PARTS and the second showing the relation between PARTS, ORDER_PARTS and SALES_HISTORY. The many-to-many relationship is implemented through the ORDER_PARTS segment which occurs in both the hierarchies. In practice, only one tree stores the ORDER_PARTS segment, while the other has a logical pointer to this segment. IMS (Information Management System) of IBM is an example of a Hierarchical DBMS.

  • In the Network Model, a parent can have several children and a child can also have many parent records. Records are physically linked through linked-lists. IDMS from Computer Associates International Inc. is an example of a Network DBMS.

  • In the Relational Model, unlike the Hierarchical and Network models, there are no physical links. All data is maintained in the form of tables consisting of rows and columns. Data in two tables is related through common columns and not physical links or pointers. Operators are provided for operating on rows in tables. Unlike the other two type of DBMS, there is no need to traverse pointers in the Relational DBMS. This makes querying much more easier in a Relational DBMS than in the the Hierarchical or Network DBMS. This, in fact, is a major reason for the relational model to become more programmer friendly and much more dominant and popular in both industrial and academic scenarios. Oracle, Sybase, DB2, Ingres, Informix, MS-SQL Server are few of the popular Relational DBMSs.

    CUSTOMER
    CUST. NO. CUSTOMER NAME ADDRESS CITY
    15371 Nanubhai & Sons L. J. Road Mumbai
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    CONTACTS
    		ORDERS
    CUST.NO. CONTACT DESIGNATION ORDER NO. ORDER DATE CUSTOMER NO.
    15371 Nanubhai Owner 3216 24-June-1997 15371
    15371 Rajesh Munim Accountant
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    PARTS ORDERS-PARTS
    PARTS NO. PARTS DESC PART PRICE ORDER NO. PART NO. QUANTITY
    S3 Amkette 3.5" Floppies 400.00 3216 C1 300
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    		 3216 S3 120
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    SALES-HISTORY
    PART NO. REGION YEAR UNITS
    S3 East 1996 2000
    S3 North 1996 5500
    S3 South 1996 12000
    S3 West 1996 20000

The recent developments in the area have shown up in the form of certain object and object/relational DBMS products. Examples of such systems are GemStone and Versant ODBMS. Research has also proceeded on to a variety of other schemes including the multi-dimensional approach and the logic-based approach.

3-Level Database System Architecture

  • The External Level represents the collection of views available to different end-users.
  • The Conceptual level is the representation of the entre information content of the database.
  • The Internal level is the physical level which shows how the data data is stored, what are the representation of the fields etc.

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